1. Field of the Invention
The present invention relates to an air conditioning system and method for vehicles, and more particularly to an air conditioning system and method suitable for use in a mobile crane vehicle.
2. Description of the Prior Art
Generally in a work vehicle such as a mobile crane, the cabin is rotatable through an angle of 360 .degree.. Consequently, the cooling water for the engine circulating below the cabin cannot be introduced into the cabin, and therefore cannot be used to heat the cabin. Nonetheless, since work vehicles typically have extensive hydraulic circuits, the cabin can utilize the heat of the hydraulic oil in the hydraulic circuit to provide heat thereto.
A typical prior art hydraulic circuit includes an oil tank and a hydraulic pump driven by an engine of the vehicle. The hydraulic pump draws oil from the oil tank and delivers the oil to a hydraulic motor which is driven by the pumped oil. The heating circuit has another hydraulic pump connected to and driven by the hydraulic motor. A valve mechanism (a relief valve) and a solenoid valve are disposed downstream of the hydraulic pump in parallel to each other in the heating circuit.
When the solenoid valve is closed, the hydraulic oil from the hydraulic pump is sent to the valve mechanism. The valve mechanism causes a pressure difference in the hydraulic oil and the hydraulic oil is heated by the pressure difference. A radiator is disposed downstream of the valve mechanism. The heat of the hydraulic oil heated by the pressure difference is radiated by the radiator. Consequently, when in the heating mode, the cabin is heated by the heat radiated from the radiator.
Additionally, the hydraulic circuit can be employed to drive an air conditioning compressor. The typical refrigerating circuit has a compressor, a condenser and an evaporator. The compressor is selectively connected to the hydraulic motor via a clutch mechanism. When the compressor is connected to the hydraulic motor, the compressor is driven and the air conditioning (cooling) is performed.
Further, when a dehumidifying and heating operation is performed, the compressor is connected to the hydraulic motor by closing the clutch mechanism and the solenoid valve in the heating circuit is closed. Thus, both the refrigerating circuit and the heating circuit are operated, and dehumidifying and heating operations are simultaneously performed.
In such a system, when the clutch to the compressor is disengaged and the operation of the heating circuit is stopped, the solenoid valve is opened. Consequently, the hydraulic oil of the heating circuit circulates between the hydraulic pump and the oil tank. Since, in such a state, the hydraulic oil is not supplied to the valve mechanism, the radiator does not radiate the heat of the hydraulic oil. Therefore, the temperature of the hydraulic oil continues to rise, often to an excessive degree.
Additionally, in conventional work vehicles, flushing the heating circuit, that is, the process for removing foreign material from the hydraulic oil is problematic and complicated. For example, in conventional systems, flushing is typically performed by detaching the valve (relief valve) and bypassing the oil path. In addition to the loss of time associated with detaching and reattaching the valve, leakage of the hydraulic oil occurs during flushing.